Peter Zijlstra wrote:
On Mon, 2009-01-26 at 08:39 -0800, Darren Hart wrote:
diff --git a/kernel/futex.c b/kernel/futex.c
index f89d373..f4132ab 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -929,7 +929,7 @@ out_unlock:
/* drop_futex_key_refs() must be called outside the spinlocks. */
while (--drop_count >= 0)
- drop_futex_key_refs(&key1);
+ drop_futex_key_refs(&key2);
Unfortunately, I realized later that this code was indeed correct and I
asked Ingo to pull my patch implementing the above change. Quoting my
previous mail on the subject:
"I believe what is happening here is that the requeue loop requeues each
waiter from one futex (key1) to another (key2). It rightly takes a
reference to the futex at key2 and then decrements the references to
key1 by drop_count (since the waiters now reference key2, not key1).
The newly taken key2 references will be dropped in futex_wait() when
each waiter is woken up and takes the futex."
Argh, that wants a comment..
Agreed. How about this.
futex: comment requeue key reference symmantics
From: Darren Hart <dvhltc@xxxxxxxxxx>
We've tripped over the futex_requeue drop_count refering to key2
instead of key1. The code is actually correct, but is non-intuitive.
This patch adds an explicit comment explaining the requeue.
Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>
---
kernel/futex.c | 7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index 8af1002..3655cad 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -969,7 +969,12 @@ out_unlock:
if (hb1 != hb2)
spin_unlock(&hb2->lock);
- /* drop_futex_key_refs() must be called outside the spinlocks. */
+ /*
+ * drop_futex_key_refs() must be called outside the spinlocks. During
+ * the requeue we moved futex_q's from the hash bucket at key1 to the
+ * one at key2 and updated their key pointer. We no longer need to
+ * hold the references to key1.
+ */
while (--drop_count >= 0)
drop_futex_key_refs(&key1);
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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